1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, facsimile device, printer, plotter, complex machine and the like.
2. Description of the Related Art
The technologies of image forming apparatuses of recent years have been developing toward high-speed/high-quality image forming apparatuses and remarkably noted because of the high stability of, particularly, the image density. In order to stabilize the image density, it is necessary to perform a suitable control on toner replenishment, and recently a method of performing such control has been a significant issue.
As the means for calculating the replenishment amount by means of the toner replenishment control, there are two methods: a method of calculating, in a pseudo manner, a toner density from mainly a sensor value to replenish a toner (sensor replenishment control); and a method of converting the number of write pixels, image-area ratio, or other write information to the amount of toner consumption, and replenishing the toner by the obtained consumption amount (pixel replenishment control).
The pixel replenishment control is a control method for replenishing the toner by the amount of output pixels (the amount of the consumed toner), and thus has an advantage that the toner replenishment amount can be calculated relatively accurately. However, the amount of toner which is actually used for development is a cause of errors in the ratios of line images/solid images, the ratio of vertical line images, and the ratio of the horizontal line images, and such errors accumulate gradually when printing a plurality of pages, thus it is difficult to constantly control the image density by means of only the replenishment control (pixel ratio control) of the toner proportional to the image-area ratio.
On the other hand, as the sensor replenishment control, there is known a replenishment control method which uses a density sensor utilizing the changes in the permeability of a developer inside a developer container, the changes being caused by the toner density. However, as described in a number of conventional technologies, this control method has changing factors such as reduction of sensor outputs caused by air stirring of the developer or increase of sensor outputs which is caused because the apparatus was left untouched for a long period of time. Therefore, in the case in which such an error is considerably big in the density variation, if a toner is replenished directly without carrying out a correction such as upper limit processing or the like, density variation occurs.
Therefore, as the toner replenishment control method, it is general to use a control method in which the pixel replenishment control and the sensor replenishment control are combined.
However, even in this simultaneous control of both pixel replenishment control and sensor replenishment control, the replenishment amount may fluctuate. For example, when outputting an image having a high image density (high image-area) or the like, control for replenishing a toner at once acts from the both pixel replenishment control and sensor replenishment control. At this moment, the replenishment amount may become excess, causing a drawback such as toner scattering or the like.
For example, in an image forming apparatus of a type in which a digital latent image is formed by using a laser scanner, LED array or the like, toner consumption amount per page can be estimated relatively accurately from a cumulative total value of the number of printing pixels in an image information signal per page. In the case in which automatic toner replenishment control is performed by a system for determining the tone replenishment amount in response to this estimated consumption amount, when one or more images with a printing ratio of as high as 80% or more are outputted during the automatic toner replenishment control, a toner is supplied at once, and development may be executed without sufficiently charging the toner, causing fogging or toner scattering. In view of such a conventional technology, Japanese Patent Application Laid-Open No. 2003-316144, for example, discloses an example of developer density control for performing a control in which an upper limit value and lower limit value are determined for the replenishment amount of a toner to be supplied at single toner replenishment, and, when a calculated replenishment amount exceeds the upper limit value, the excess amount is carried over to the next toner replenishment amount, while, when the calculated replenishment amount does not reach the lower limit value, the replenishment amount is carried over to the next toner replenishment amount.
In this known control technology, the upper limit of the replenishment amount is determined based on the replenishment amount of a high printing ratio image, thus this technology is effective to cope with problems occurring at the time of replenishment for the high printing ratio image. However, since the upper limit value is determined uniquely (to a fixed value) regardless of the image printing ratio, it can be fully expected that a problem is caused by the excessive replenishment amount at the time of replenishment for a low printing ratio image. When considering that the amount of toner to be replenished (=consumption amount in printing) is proportional to an image-area ratio, the above fact is based on the idea that the upper limit value for the replenishment amount, which is set in view of the above-described causes of error, fluctuation, and fluctuation of actual replenishment amount, should also be made proportional to the image-area ratio to a certain extent.
On the other hand, regarding the toner replenishment control, Japanese Patent Application Laid-Open No. 2005-77622 discloses a technology in which, in the case where a toner replenishment amount, which is acquired based on the amount of toner consumed in formation of a toner image, exceeds a predefined replenishment lower limit value, and where this controlled amount of toner exceeds a replenishment upper limit value which is an upper limit value of the amount of toner replenished at single replenishment operation, a toner in the amount of the replenishment upper limit value is replenished, and in the case where the toner replenishment amount, which is acquired based on the amount of toner consumed in formation of a toner image, exceeds the predefined replenishment lower limit value, and where this toner replenishment amount is below the replenishment upper limit value, this toner replenishment amount is replenished. This patent application discloses that the toner replenishment upper limit value is appropriately set in response to the amount of change in an average toner density which is acquired based on image data.
In addition, in the pixel replenishment control, in an output image the number of pixels in a line image is same as the number of pixels in a solid image. However, since the consumption amount of toner is different in both line and solid images, if the number of line images is high, the toner consumption amount is large in the line images than in the solid image. The reason is because the adhesion amount of the toner becomes higher in the line image than in the solid part because of the edge effect of a latent image.
Furthermore, in recent image forming apparatuses, the diameter of particles in a developer has been gradually reduced in order to obtain high-quality images. Particularly in two-component development, reduction of the diameter of particles in a carrier worsens the liquidity of the developer. For this reason, even when a toner for replenishment is added, it is not easily mixed with the carrier, and when a bit more toner is replenished, a toner which is not mixed with the carrier is generated, thus there is a high possibility that toner scattering is caused and the image quality is deteriorated. Specifically, more appropriate replenishment control needs to be performed for reduction of the diameter of particles in the developer.